1. Field of the Invention
This invention relates tetramanganese-oxo cubane complexes, their synthesis and their use as a functional catalyst for in vitro O2 production. The compound of this invention has a cubical shaped core comprised of 4 Mn atoms and 4 O atoms and is supported by a surrounding matrix of six facially bridging chelates, preferably phosphinates (RRxe2x80x2PO2), although other chelates are possible. The complexes of this invention are useful as catalysts for such industrially important reactions as the oxidation of chloride to chlorine, the oxidation of water to hydrogen peroxide and oxygen and oxygeneration and oxidation reactions of hydrocarbons and organic substrates.
2. Related Art
The production of molecular oxygen by photosynthesis is a dominant global process that replenishes the atmospheric O2 that is essential to sustain all aerobic life on earth. To date, until this invention, there does not exist a functional a functional catalyst for in vitro O2 production. The use of such a catalyst for water oxidation would make available large quantities of oxygen needed to replace air for environmentally-clean fuel combustion and remote O2 needs. The positive impact that such technology would have on improving health has global proportions for all respiring organisms, particularly those affected by respiratory diseases or dysfunctions.
More particularly, much of the industrialized world""s air pollution comes from internal combustion engines, e.g., automobiles, trucks, diesal engines, etc. Such combustion engines produce NOx gases due to the use of air instead of pure oxygen for combustion. These gases are an essential causative agent in the formation of ozone, photochemical smog and atmospheric aerosols. Hospital visits for asthma and other respiratory disorders have been directly correlated with these pollutants. Ice core records from the South Pole indicate that N2O in the atmosphere has increased by 9% in the last 40 years (a much higher rate than in the previous 60 years), and continues to grow at an increasing rate.
Hydrogen peroxide, oxygen and chlorine are bulk chemicals used in large amounts in many industrial processes. For example, Chem. Engineering News, Jun. 23, 1997, p. 42, reported that in 1996, 355,000 tons of hydrogen peroxide (100%), 668,000 tons of oxygen, and 12,621,000 tons of chlorine were produced. Oxygen is currently obtained from air by separation using membranes and by liquification/distillation. Peroxide is made commercially by electrochemical reduction of oxygen catalyzed by quinones. It would be highly desirable to have a more economic means of producing such chemicals.
As indicated previously, there are no commercially viable man-made homogeneous catalysts for oxygen production by water oxidation. Such a commercially viable catalyst has many uses. For example, it could be used for the photochemical or electrochemical generation of O2 from water to improve the efficiency of industrial combustion processes and automobile engines, while helping to solve a major air pollution problem.
It is known that photosynthetic oxygen evolution by water oxidation is catalyzed by a metalloenzyme that contains the same invariant inorganic core in all plants, comprised of a spin-coupled tetramanganese cluster, 1 Ca2+ ion and an unknown number of Cl-ions. This water oxidizing complex (WOC) is an integral part of the Photosystem II (PSII) reaction center protein complex. The structure of the tetrameric Mn core has only been incompletely established, but has two Mnxe2x80x94Mn separations of 2.7 xc3x85 and 3.3 xc3x85 as seen by EXAFS. The WOC can be photooxidized by the PSII reaction center to produce five oxidation states, differing by one electron each and designated, S0, S1, . . . S4.
More particularly, the inorganic core of the photosynthetic water oxidase (WOC) is comprised of a cluster of composition Mn4OxCa1Cly, in which only the stoichiometry of Mn and Ca are well established, Debus, R. J., Biochim. Biophys. Acta 1992, 1102, 269-352; Yachandra, V. K.; Souer, K.; Klein, M. P., Chem. Rev. 1996, 96, 2927-2950; Zaltsman, L.; Ananyev, G.; Bruntrager, E.; Dismukes, G. C., Biochemistry 1997, 36, 8914-8922; Ananyev, G. M.; Dismukes, G. C., Biochemistry 1997, 36, 11342-11350). Comparisons to Mn-oxo complexes have played a key role in suggesting possible core types such as the dimer-of-dimers, Yachandra, V. K., Sauer, K., Klein, M. P., Chem. Rev. 1996, 96, 2927-2950; Kirk, M. L., Chan, M. K., Armstrong, W. H., Solomon, E. I., J. Amer. Chem. Soc. 1992, 114, 10432-10440) (Mn2O2)O(Mn2O2)n+, trigonal, Wang, S., Tsai, H. -L., Hagen, K. S., Hendrickson, D. N., Christou, G., J. Am. Chem. Soc. 1994, 116, 8376-8377) Mn4O3Xn+, distorted cubane, Vincent, J. B., Christou, G., Inorg. Chim. Acta 1987, 136, L41-L43; Mn4O4n+, adamantane Brudvig, G. W., Crabtree, R. H., Proc. Natl. Acad. Sci. USA 1986, 83m 4586-4588; Wieghardt, K., Angew. Chem. (Int. Ed. Engl.) 1989, 28, 1153-1172; Mn4O6n+, and butterfly Vincent, J. B., Christou, G., Inorg. Chim. Acta 1987, 136, L41-L43) Mn4O2n+.
Structurally characterized Mn compounds have been reported to catalyze water oxidation. For example, Mn6-silesquioxane, is a hexanuclear cluster is a heterogeneous catalyst with low activity and stability. Another is a covalently-linked dimangano-porphrin complex that contains a biologically unavailable fluorinated ligand and is active for a few turnovers. Another compound is comprised of a demanganese-di-xcexc-oxo core with terminal tridentate chelate (terpyridyl) and water, Mn2O2 (terpy)2(H2O), Limburg. et al., (1999) Science, Vol. 283, 1524-1527. To date, no stab tetramanganese Mn complex of any type has been synthesized that exhibits four-electron redox chemistry, a hallmark of the WOC.
Multinuclear manganese-oxo clusters have been synthesized for use as molecular magnets, (Aubin, S. M. J.; Dilley, N. R.; Pardi, L.; Krzystek, J.; Wemple, M. W.; Brunel, L. -C.; Maple, M. B.; Christou, G.; Hendrickson, D. N., J. Am. Chem. Soc. 1998, 120, 4991-5004; Aubin, S. M. J.; Wemple, M. W.; Adams, D. M.; Tsai, H. -L.; Christou, G.; Hendrickson, D. N., J.Am. Chem. Soc. 1996, 118, 7746-7754), oxidation catalysts, (Gardner, K. A.; Mayer, J. A., Science 1995, 269, 1849-1851) and bioinorganic compounds for photosynthetic water oxidation, (Brudvig, G. W.; Crabtree, R. H., Proc. Natl. Acad. Sci. USA 1986, 83, 4586-4588; Vincent, J. B., Christou, G., Inorg. Chim. Acta 1987, 136, L41-L43; Wieghardt, K., Angew. Chem. (Int. Ed. Engl.) 1989, 28, 1153-1172; Wieghardt, K., Angew Chemie, Int.Ed 1994, 33, 725-726. Christou, G., Acc. Chem. Res. 1989, 22, 328-335; Armstrong, W. In Manganese Redox Enzymes; V. L. Pecoraro, Ed., VCH: New York, 1992; pp 261-286; Pecoraro, V. L.; Baldwin, M. J.; Gelasco, A., Chem. Rev. 1994, 94, 807-826; Watkinson, M.; Whiting, A.; McAuliffe, C. A., J. Chem. Soc., Chem. Commun. 1994, 2141-2142).
The structure of the molecule of this invention was described by the inventors in a paper entitled Synthesis and Characterization of Mn1O1L6 Complexes with Cubane-Like Core Structure: A New Class of Models of the Active site of the Photosynthetic Water Oxidase, W. Ruettinger, C. Campana, C. Dismukes (1997) J. American Chemical Society, Vol. 119, No. 28, pp. 6670-6671, July, 1997.
This invention is directed to novel, highly reactive, stable tetramanganese-oxo cubane complexes, their synthesis and their use as a functional catalyst for in vitro O2 production. Preferred complexes are i) a tetramanganese-oxo cubane complex having the formula L6Mn4O4, wherein Mn4O4, is a cubane core and L6 are six facially-capping bidentate ligands bridging between the Mn atoms; ii) a manganese-oxo-pyramid complex having the formula L6Mn4O3, wherein Mn4O3, is a pyramidal core and L6 are six bidentate ligands bridging between the Mn atoms; and iii) a manganese-oxo-butterfly complex having the formula L6Mn4O2 or L5Mn4O2, wherein Mn4O2, is a butterfly core and L6 or L5 are six or five bidentate ligands bridging between the Mn atoms. Preferred ligands are carboxylate, phosphinate or diphenylphosphinate ligands.
A highly preferred complex of this invention has a cubical shaped core comprised of 4 Mn atoms and 4 O atoms and is supported by a surrounding matrix of six facially bridging chelates, preferably phosphinates (RRxe2x80x2PO2), although other chelates are possible. This inorganic catalyst has been found to have a cubical core (Mn4O4)(Mn4O4). Two distinct oxidation states have been found useful as catalysts, and include the xe2x80x9coxidizedxe2x80x9d member of the Mn4O4-Cubane family described in the aforementioned Synthesis and Characterization of Mn4O4L6 Complexes with Cubane-Like Core Structure: A New Class of Models of the Active site of the Photosynthetic Water Oxidase, W. Ruettinger, C. Campana, C. Dismukes (1997) J. American Chemical Society, Vol. 119, No. 28, pp. 6670-6671, July, 1997 and Ruettinger. et al., (1999) Inorganic Chemistry, the entire disclosure of which is incorporated herein by reference.
These catalysts may be used for the oxidation of water to oxygen and hydrogen peroxide and may also be used to catalyze the oxidation of chloride and simple chloride compounds to chlorine gas. The catalysts are capable of both two-electron and four-electron oxidation reactions with or without dehydrogenation.
Additionally, due to the catalytic properties of these novel complexes they may be useful in fuel cell reactions, welding by, for example, automobile manufacturers, steel industry, chemical synthesis, pulp and paper bleaching, fertilizers, sewage treatment, bleaching applications, hospitals, bulk chemical commodities (O2, H2O2, Cl2), specialty chemical synthesis. These catalysts have characteristics similar to the photosynthetic enzyme which produces oxygen.
Synthesis of the cubical core is a two-step process having yields as high as 80% and comprises the ligand-assisted fusion of, for example, two Mn2O23+ cores. In order to enhance the commercialization prospects for these catalysts it is desirable to have a means to regenerate (reoxidize) these compounds.